System and method for stimulating the heart via the vagus nerve

ABSTRACT

A rules engine acquires sensor data from sensors applied to the heart to determine the presence of a rapid heartbeat. The rules engine applies rules to the sensor data to determine whether to deliver an electrical waveform to a vagus nerve. The rules engine further determines whether an electrical waveform should be applied to the heart and, if so, the type of electrical waveform.

RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalPatent Application No. 61/826,704, entitled “System and Method forStimulating the Heart via the Vagus Nerve” filed May 23, 2013, theentire contents of which are hereby incorporated by reference for allpurposes.

BACKGROUND

The heart is divided into the right side and the left side. The rightside, comprising the right atrium and ventricle, collects and pumpsde-oxygenated blood to the lungs to pick up oxygen. The left side,comprising the left atrium and ventricle, collects and pumps oxygenatedblood to the body. Oxygen-poor blood returning from the body enters theright atrium through the vena cava. The right atrium contracts, pushingblood through the tricuspid valve and into the right ventricle. Theright ventricle contracts to pump blood through the pulmonic valve andinto the pulmonary artery, which connects to the lungs. The blood picksup oxygen in the lungs and then travels back to the heart through thepulmonary veins. The pulmonary veins empty into the left atrium, whichcontracts to push oxygenated blood into the left ventricle. The leftventricle contracts, pushing the blood through the aortic valve and intothe aorta, which connects to the rest of the body. Coronary arteriesextending from the aorta provide the heart blood.

The heart's own pacemaker is located in the atrium and is responsiblefor initiation of the heartbeat. The heartbeat begins with activation ofatrial tissue in the pacemaker region (i.e., the sinoatrial (SA) node),followed by cell-to-cell spread of excitation throughout the atrium. Theonly normal link of excitable tissue connecting the atria to theventricles is the atrioventricular (AV) node located at the boundarybetween the atria and the ventricles. Propagation takes place at a slowvelocity, but at the ventricular end the bundle of His (i.e., theelectrical conduction pathway located in the ventricular septum) and thebundle braides carry the excitation to many sites in the right and leftventricle at a relatively high velocity of 1-2 m/s. The slow conductionin the AV junction results in a delay of around 0.1 seconds betweenatrial and ventricular excitation. This timing facilitates terminalfilling of the ventricles from atrial contraction prior to ventricularcontraction. After the slowing of the AV node, the bundle of Hisseparates into two bundle branches (left and right) propagating alongeach side of the septum. The bundles ramify into Purkinje fibers thatdiverge to the inner sides of the ventricular walls. This insures thepropagation of excitatory waveforms within the ventricular conductionsystem proceeds at a relative high speed when compared to thepropagation through the AV node.

When the heart is working properly, both of its lower chambers(ventricles) pump at the same time as, and in synchronization with, thepumping of the two upper chambers (atria). Up to 40 percent of heartfailure patients, however, have disturbances in the conduction ofelectrical impulses to the ventricles (e.g., bundle branch block orintraventricular conduction delay). As a result, the left and rightventricles are activated at different times. When this happens, thewalls of the left ventricle (the chamber responsible for pumping bloodthroughout the body) do not contract simultaneously, reducing theheart's efficiency as a pump. The heart typically responds by beatingfaster and dilating. This results in a vicious cycle of furtherdilation, constriction of the vessels in the body, salt and waterretention, and further worsening of heart failure. These conductiondelays do not respond to antiarrhythmics or other drugs.

Patients who have heart failure may be candidates to receive apacemaker. A pacemaker is an artificial device to electrically assist inpacing the heart so that the heart may pump blood more effectively.Implantable electronic devices have been developed to treat bothabnormally slow heart rates (bradycardias) and excessively rapid heartrates (tachycardias). The job of the pacemaker is to maintain a safeheart rate by delivering to the pumping chambers appropriately timedelectrical impulses that replace the heart's normal rhythmic pulses. Thedevice designed to perform this life-sustaining role consists of a powersource the size of a silver dollar (containing the battery), and controlcircuits, wires or “leads” that connect the power source to the chambersof the heart. The leads are typically placed in contact with the rightatrium or the right ventricle, or both. They allow the pacemaker tosense and stimulate in various combinations, depending on where thepacing is required.

Either cathodal or anodal current may be used to stimulate themyocardium. The pulses produced by most pacemakers are typicallycathodal and excitatory. That is, the cathodal pulse is of sufficientmagnitude and length to cause the heart to beat. Cathodal currentcomprises electrical pulses of negative polarity. This type of currentdepolarizes the cell membrane by discharging the membrane capacitor, anddirectly reduces the membrane potential toward threshold level. Cathodalcurrent, by directly reducing the resting membrane potential towardthreshold has a one-half to one-third lower threshold current in latediastole than does anodal current.

Anodal current comprises electrical pulses of positive polarity. Theeffect of anodal current is to hyperpolarize the resting membrane. Onsudden termination of the anodal pulse, the membrane potential returnstowards resting level, overshoots to threshold, and a propagatedresponse occurs. The use of anodal current to stimulate the myocardiumis generally discouraged due to the higher stimulation threshold, whichleads to use of a higher current, resulting in a drain on the battery ofan implanted device and impaired longevity. Additionally, the use ofanodal current for cardiac stimulation was discouraged due to thesuspicion that the anodal contribution to depolarization can,particularly at higher voltages, contribute to arrhythmogenesis.

It has been shown that pacing in which a combination of cathodal andanodal pulses of either a stimulating or conditioning nature preservesthe improved conduction and contractility of anodal pacing whileeliminating the drawback of increased stimulation threshold. The resultis a depolarization wave of increased speed. This increased propagationspeed results in superior cardiac contraction leading to an improvementin blood flow. Improved stimulation at a lower voltage level alsoresults in reduction in power consumption and increased life forpacemaker batteries.

Vagal nerves innervate the sinus node, the atrioventricular conductingpathways and the atrial muscle. Stimulation of either vagus nerve slowsthe heart by its effect on the above-mentioned structures. The latencyof the response of the sinus node is very short, and the effect of asingle vagal impulse depends on the phase of the cardiac cycle duringwhich it is applied. Thus, vagal stimulation results in a peak responseeither in the first or the second beat after its onset. The slowing ofheart rate increases with the frequency of vagal stimulation and therelationship of this to pulse interval is linear.

Sympathetic postganglionic fibers innervate the entire heart, includingthe sinus node, the AV conducting pathways and the atrial andventricular myocardium. Increased activity of the sympathetic nervesresults in increase in heart rate and force of contraction. In addition,the rate of conduction through the heart is increased and the durationof contraction is shortened. When sympathetic activity increases, thereis a latent period of up to five seconds before an increase in heartrate, which reaches a steady level after about 30 seconds. This is inmarked contrast to vagal effects, which are almost instantaneous.

The frequency of sympathetic activity is also linearly related to heartperiod with the right sympathetic nerve having a greater effect on sinusnode rate than the left sympathetic nerve.

SUMMARY

In an embodiment, a memory is configured to store one or more anodalwaveforms, cathodal waveforms, and biphasic waveforms. A waveform or acombination of waveforms may be selected from the memory by a processorbased on sensor data, data about the user and rules also stored in thememory. The stored waveforms comprise waveform data that are used by themulti-phase cardiac stimulus generator to produce waveforms for applyingto the heart and/or the vagus nerve.

DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitutepart of this specification, illustrate exemplary embodiments of theinvention, and together with the general description given above and thedetailed description given below, serve to explain the features of theinvention.

FIG. 1 is a schematic representation of the electrical activity of atypical heartbeat as is known in the prior art.

FIG. 2 is a schematic representation illustrating a cardiac stimulationdevice according to an embodiment.

FIG. 3 is a schematic representation illustrating a cardiac and vagusnerve stimulation device according to an embodiment.

DETAILED DESCRIPTION

As used herein, the term “pulse” refers to a single occurrence of anelectrical signal that has a defined shaped and period.

As used herein, the term “waveform” refers to a repeating electricalsignal that may include one or more pulses. The pulses that make up thewaveform may be the same or may differ in any one of shape, polarity,duration and amplitude. For example, a biphasic waveform may include ananodal pulse and a cathodal pulse. The anodal and cathodal componentsmay differ only in polarity or may be differ in shape, polarity,duration and amplitude. Pulses making up a waveform may differ in shape,polarity, duration, and amplitude but be equivalent in power.

As used herein, the term “sub-threshold waveform” refers to a waveformthat does not result in stimulating the heart to beat. A waveform may besub-threshold because the amplitude of the waveform is below anamplitude threshold value necessary to stimulate a heartbeat. A waveformmay be sub-threshold because the duration of the waveform is below aduration threshold value necessary to stimulate a heartbeat. A waveformmay be sub-threshold because the power of the waveform is below a powerthreshold value necessary to stimulate a heartbeat.

As used herein, the term “pacing waveform” refers to a waveform thatstimulates a heartbeat, results in depolarization and is by definitionequal to or greater than a threshold necessary to simulate a heartbeat.

FIG. 1 shows a representative tracing 10 of electrical activity in atypical heartbeat. A P wave 11 represents the wave of depolarizationthat spreads from the SA node throughout the atria. A period of timefrom the onset of the P wave to the beginning of a QRS complex is knownas the P-R interval 12. The P-R interval 12 represents the time betweenthe onset of atrial depolarization and the onset of ventriculardepolarization (typically lasting 20-200 ms). If the P-R intervalis >200 ms, there is an AV conduction block, which is also known as afirst-degree heart block if the impulse is still able to be conductedinto the ventricles.

A QRS complex 13 represents the period of ventricular depolarization,which normally occurs very rapidly (e.g., typically lasting 80-120 ms).If the QRS complex is prolonged, conduction is impaired within theventricles.

The isoelectric period (ST segment 14) following the QRS complex 13 isthe period of time (typically lasting 80-120 ms) at which the entireventricle is depolarized and roughly corresponds to the plateau phase ofthe ventricular action potential. The ST segment 14 is important in thediagnosis of ventricular ischemia or hypoxia because under thoseconditions, the ST segment 14 can become either depressed or elevated.

FIG. 2 is a schematic representation illustrating a multi-phase cardiacstimulus generator 120 implanted in a patient according to anembodiment. In an embodiment, one or more sensors sense rhythm andcontractions of the patient's heart 105 using at least one of atrialsensing and ventricular sensing, such as at least one of atrial sensor110 and ventricular sensor 112. The atrial sensor 110 and/or ventricularsensor 112 provide sensor data to a rules engine 122. In an embodiment,the rules engine includes a processor 126 and a memory 124 for storingrules and receiving sensor data. The rules engine 122 may poll the oneor more of the atrial sensor 110 and the ventricular sensor 112 toobtain sensor data and to apply the rules to the sensor data in order todetermine whether to deliver electrical waveforms to one or moreelectrodes, and, if electrical waveforms are to be delivered, which ofthe one or more electrodes is to receive the electrical waveforms. In anembodiment, the one or more electrodes may be an atrial electrode 114and a ventricular electrode 116, and may provide electrical waveforms toat least one of an atrial chamber and a ventricular chamber of the heart105. The multi-phase cardiac stimulus generator 120 may generate ananodal waveform, a cathodal waveform, and a biphasic waveform above orbelow threshold depending on the sensor data and the rules applied bythe rules engine 122.

In an embodiment, the memory 124 of the rules engine 122 of themulti-phase cardiac stimulus generator 120 is configured to store one ormore anodal waveforms, cathodal waveforms, and biphasic waveforms. Awaveform or a combination of waveforms may be selected from the memory124 by the processor 126 based on sensor data and based on rules alsostored in memory 124.

In an embodiment, the memory 124 may also store information about thepatient 100. The processor 126 may further select a waveform or acombination of waveforms from the stored waveforms based on the sensordata and data about the user.

In an embodiment, the stored waveforms comprise waveform data that areused by the multi-phase cardiac stimulus generator 120 to producewaveforms for applying to the heart.

FIG. 3 is a schematic representation illustrating a cardiac and vagusnerve stimulation device according to an embodiment. FIG. 3 includeselements from FIG. 2 and additionally includes a vagus electrode 310 forstimulating the vagus nerve 305. In an embodiment, the vagus nerve 305is accessed in the neck. The carotid sheath is dissected and the vaguselectrode 310 is an encircling electrode that is wrapped around thevagus nerve 305 to receive electrical stimulation.

In an embodiment of the present invention, the one or more of atrialsensor 110 and ventricular sensor 112 provide sensor data that indicatesthe onset of tachycardia. In response to the heart sensor data, therules engine 122 may apply an electrical waveform to the vagus nerve 305via the vagus electrode 310. The rules engine 122 monitors the sensordata to determine when a sinus rhythm has been reestablished in thecardiac tissue. If a sinus rhythm has been reestablished in the cardiactissue, the rules engine 122 halts the stimulus to the vagus electrode310. If a sinus rhythm has not been reestablished in the cardiac tissue,the rules engine 122 continues the stimulation of the vagus nerve 305.

In an embodiment, the electrical waveform may be low frequency or highfrequency electrical signal or a waveform made up of trains ofelectrical pulses. In an embodiment, the electrical waveform is abiphasic waveform. In another embodiment, the stimulation of the vagusnerve may be combined with the application of a sub-threshold waveformto the heart as previously described.

A system and method for reducing stroke work in an artificially pacedheart have been disclosed. It will also be understood that the inventionmay be embodied in other specific forms without departing from the scopeof the invention disclosed and that the examples and embodimentsdescribed herein are in all respects illustrative and not restrictive.Those skilled in the art of the present invention will recognize thatother embodiments using the concepts described herein are also possible.Further, any reference to claim elements in the singular, for example,using the articles “a,” “an,” or “the” is not to be construed aslimiting the element to the singular.

What is claimed is:
 1. An apparatus configured to apply a therapy to apatient's heart comprising: a multi-phase cardiac stimulus generator;one or more sensors; one or more electrodes configured to be coupled toat least one of an atrial chamber and a ventricular chamber of theheart; one or more electrodes configured to encircle and contact a vagusnerve; a processor; and a memory configured to store data from the oneor more sensors, waveform data, and rules that when executed by theprocessor cause the processor to detect using the one or more sensors arapid heartbeat; apply the rules to the sensor data to determine whetherto deliver a first electrical waveform to the vagus nerve; selectwaveform data from the memory corresponding to the first electricalwaveform; deliver the first electrical waveform, based on the waveformselected from the memory, to the one or more electrodes configured toencircle and contact the vagus nerve; determine whether to deliver asub-threshold electrical waveform to the heart via at least one of theone or more electrodes; instruct the multi-phase cardiac stimulusgenerator to generate the sub-threshold electrical waveform from thestored waveform data when it is determined to deliver the sub-thresholdelectrical waveform to at least one of the one or more electrodes; anddeliver the sub-threshold electrical waveform to the one or moreelectrodes that are coupled to the at least one of an atrial chamber anda ventricular chamber of the heart.
 2. The apparatus of claim 1, whereinthe first electrical waveform is a biphasic waveform.
 3. The apparatusof claim 1, wherein the sub-threshold electrical waveform is an anodalor biphasic waveform.
 4. The apparatus of claim 1, wherein the memory isfurther configured to store patient data, and the rules further causethe processor to select the waveform data from the memory based, atleast in part, on the patient data.
 5. The apparatus of claim 1, whereinthe rules further cause the processor to determine whether to deliverthe sub-threshold electrical waveform based on whether the one or moresensors indicate that a sinus rhythm has been reestablished in thepatient's heart.
 6. The apparatus of claim 1, wherein rules furthercause the processor to deliver a stimulating electrical waveform to theone or more electrodes configured to encircle and contact the vagusnerve while delivering the sub-threshold electrical waveform to one ormore of the electrodes configured to be coupled to at least one of theatrial chamber and the ventricular chamber of the heart.
 7. Theapparatus of claim 1, wherein at least one of the first electricalwaveform and the sub-threshold electrical waveform includes a train ofpulses.
 8. The apparatus of claim 1, wherein the rules further cause theprocessor to detect an onset of tachycardia or other undesired cardiacrhythm based on data from the one or more sensors, and to deliver thefirst electrical waveform to the one or more electrodes configured toencircle and contact the vagus nerve when the onset of tachycardia isdetected.
 9. The apparatus of claim 1, wherein the sub-thresholdelectrical waveform is delivered while the first electrical waveform isdelivered.
 10. The apparatus of claim 1, wherein the sub-thresholdelectrical waveform is a waveform that is insufficient to cause thepatient's heart to beat.
 11. The apparatus of claim 1, wherein thesub-threshold electrical waveform is delivered after delivery of thefirst waveform is completed.